Someone was explaining this to me last week. The algorithm goes by comments and upvotes. I was seeing 3 year old posts but had upvoted before I saw how old it was. That sent it to hot and then someone probably made the same mistake and so on, which made the post get hotter. I think they're re-configuring the algorithm so it may change.

Also, this is conjecture, but I think when you're in all, you're seeing your instance's version and everyone else's version of the same post. So if someone from your instance upvotes, it makes it hotter. Please correct me if I'm wrong, I'm not positive this is the case.